1. Field of the Invention
This invention relates generally to offshore floating structures, and, more particularly to a tension-based tension leg platform (TBTLP) for use in ultra deepwater to support dry-tree oil and gas production which utilizes a tension base or artificial seabed to simplify tendon design at deep water locations in harsh environment and a truss pontoon to reduce vertical and horizontal wave loadings.
2. Background Art
Oil and gas companies have used Tension Leg Platforms (TLP) for production in deepwater since 1980 until the alternate Truss-SPAR technology was developed and tested in the Gulf of Mexico. The TLP is an excellent and reliable technology because it has proven behavior in the harsh environment of deepwater. The industry takes no risk in using this technology because several technical issues had been solved in the real world. The TLP is moored to the seabed by high tensile strength steel tubes called tendons which allow very little vertical motion to the platform. There are over twenty TLPs in operation in the Gulf of Mexico, the North Sea, offshore Indonesia and West Africa. Shell Oil Company had developed several of its deepwater oil fields with TLP technology in the Gulf of Mexico and is currently working on MARS TLP phase 2 with TLP concept in 3000 ft water depth. BHP Billiton has the intention to use a TLP for the Shenzi development in 4,300 ft deepwater of the Gulf of Mexico for 100 kBD of oil and 50 million cubic feet per day of gas. Shenzi is located 120 miles off Louisiana shore in Green Canyon Block 653.
The water depth limitation of this TLP technology is less than 5000 ft for several technical reasons that are discussed hereinafter. The engineering and construction companies that serve the oil companies designed several geometric variations to the TLP technology in order to make it cost effective in applications at water depths for the given deck pay load capacity. Of course environmental conditions are also key factors in the design of the TLP like any other offshore platforms.
The oil and gas industry has in-depth knowledge about the TLP design for the given application factors. The regulatory world also has established full depth standards for the classing of the structure. FIG. 1 shows a typical conventional TLP of the prior art with conventional hull, tendons, deck, production risers etc. FIG. 2 shows, schematically, the 2-D geometric displaced position of the conventional TLP in sway with pitch and set-down relationship. The TLP may be used as a processing and/or wellhead platform. Drilling and work-over capability may be provided with the tendon assisted rigs. Storage can be provided with an independent floating storage unit (FSU) vessel or SPAR.
Although the TLP looks like a floating vessel, in the operational mode, it behaves like a fixed structure in the heave, roll and pitch motions and thus enables support of a conventional dry-tree production system. The excess buoyancy designed in the hull with vertical columns and conventional pontoons keeps the tendons in tension to be effective in all wave environments for the given deck load. A TLP tendon requirement to keep the vessel concept effective within the practical cost budget is limited by the water depth. If the TLP's heave or pitch or roll periods become longer than 4-5 seconds, then the dynamic system is susceptible to direct wave energy at resonance. That leads to motions and severe fatigue problems. When the tendons become longer according to the water depth, they are required to be larger in size in cross section to maintain the same stiffness. At water depths close to 5000 ft, the weight of the tendons becomes impractical. The vessel size to always maintain the tendons in tension becomes uneconomically large. Several alternatives had been proposed utilizing composite materials instead of steel for the tendons which may provide weight savings. However, they are not yet practical from the standpoint of cost. The present invention introduces innovative technologies to enhance TLP applications in water depths of over 8000 ft.
Ultra Deepwater Problems
As discussed above, one of the problems in extending TLP technology to ultra deepwater applications is that the TLP is sensitive to topside deck load because it loses its pretension capacity with respect to the topside weight for the given hull buoyancy. The hull weight to total displacement ratio for the given deck load for the range of water depths that is feasible for TLP is almost same as for a semi-submersible structure. The current technology allows the TLP to be designed for water depth of 4500 ft with a topside deck payload of 20,000 tons for the Gulf of Mexico environment. The most attractive feature of the TLP is its application for dry-tree support in the production platform. Subsea well solutions although attractive in deepwater, requires well intervention when the well is not producing as expected and becomes expensive. Many wells have been abandoned because the cost-effectiveness to fix them in the deep water with subsea wells.
Thus, the TLP offers a good option for simplifying well intervention when a well develops problems or needs well enhancement. From the well maintenance cost point of view, the TLP is a better option than a semi-submersible unless it is a dry-tree support semi-submersible. As stated earlier, the TLP has a water depth limitation problem utilizing steel tendons. The vessel size is also large when the water depth increases to over 4000 ft. Consequently, the installation of the vessel becomes difficult as the water depth increases beyond 5000 ft. Stepped tendons and composite materials have been offered as a solution to the tendon engineering problem.
Water depth increases have more serious problems with the TLP applications. One of them is the riser pretension. The increased water depth increases the riser weight and thus increases the pretension. The second effect on the riser is due to the external water pressure in deepwater. This typically results in larger wall thickness for the risers. This solution further increases the riser weight and the riser pretension requirements. Ultra deepwater developments become economic if drilling support is on the deck. Table 1, below, gives a typical load estimation for a TLP in the Gulf of Mexico at an 8000 ft water depth and in drilling and production of 100 (kBD) oil applications. It should be noted that the riser pretension, the SCR and the umbilical down preloads exceed deck payload by 1.6 times in ultra deepwater. The total down load required to be supported by the TLP to work in 8000 water depth is 80,000 short tons. In addition to that, the displacement of the vessel should take care of the tendon preloads. Thus the TLP vessel size becomes unusually large and the tendons design becomes challenging, and development of a new technology is required to use TLP in ultra deepwater fields.
TABLE 1Estimation of Weight, GOM-TLPWater Depth8000 ft100 yr WaveHs = 51.8 ft & Tp = 15.2 sProduction oil, gas100 kBD & 50 mcftpdDeck Pay Load17,000 short tonsDeck Self Weight 7,000 short tonsHull Weight29,000 short tonsRiser Tension21,000 short tonsSCR + Umbilical 6,000 short tonsTotal Down Load80,000 short tons
The Shell Ursa TLP uses 14.0 inch OD and 0.75 inch wall thickness and 10⅞ inch OD and 0.608 inch wall thickness production risers for the Gulf of Mexico 4000 ft water depth. Engineering challenges are being realized for 10,000 ft water depth and 10,000 psi internal pressure for the Gulf of Mexico ultra deepwater applications in the riser design. For this design, significant increase in diameter and wall thickness of the risers is seen. Subsequently, the top tension requirement for the risers also increases. Thus, designing a TLP for ultra deepwater has three basic problems: (1) Vessel Size, (2) Tendon Design, and (3) Riser Design. The present invention meets these challenges to bring the TLP within technical feasibility and cost effectiveness for the ultra deepwater depths.